Microwaves antennas are used in a variety of medical devices to treat several medical conditions. Several conditions including cardiac electrophysiological disorders, cancer, Menorrhagia, etc. are currently treated by applying microwave energy for ablating tissue. For example, microwave antennas (e.g. helical antennas) have been used in medical applications including treatment of benign prostate hyperplasia, cancer treatment, etc. Many of the existing antennas have common disadvantages such as device shaft heating and non-uniform lesion profile along the length of the antenna.
FIG. 1B illustrates some of the common disadvantages of prior art antennas used for ablating solid tissue such as solid tumors. Devices that use microwave ablation for treating tumors are advantageous over devices that use other ablation modalities because of their potential to create relatively larger, uniform volumetric lesions that are relatively unaffected by the heat sink effect of a nearby blood vessel. Most existing microwave ablation devices are derived from known microwave antenna structures such as monopole or dipole or helical antenna and have a linear structure. Their SAR and thermal profile are substantially elliptical and they are approximately similar to the shape of a football as shown in FIG. 1B. FIG. 1B shows a photograph of the cut surface of swine liver that was ablated using a monopole microwave antenna. FIG. 1B shows an elliptical ablation wherein only a portion of the ablation extended over a microwave antenna 104 (zone Z2) and a significant portion of the ablation extended over the shaft of the transmission line 102 (zone Z1). A significant amount of microwave field is located proximal to the distal end of the coaxial cable or other transmission line feeding the radiating element (monopole antenna). Such an ablation shape is caused due to the undesired backward coupling of the microwave field emitted by the antenna to the distal region of a conductor of the transmission line (e.g. the outer conductor of a coaxial cable). This causes a significant portion of the microwave field to be located around the distal region of the transmission line as a “long tail” instead of being localized around the microwave antenna. This portion of the microwave field can damage healthy tissue and increase the morbidity of the medical procedure. Further, this portion of the microwave field can heat up the transmission line and further damage healthy tissue. In order to overcome this problem, microwave devices have been developed that comprise a cooling mechanism around the transmission line. However, this increases the outer diameter of the device thereby increasing the invasiveness of the procedure. Further, devices with a cooling mechanism need extra equipment for circulating the coolant through the cooling mechanism thereby increasing the device complexity and cost.
Referring again back to FIG. 1B, it is clear that the backward coupling causes only a fraction of the microwave field to be delivered to the target tissue. A large portion of the field is located over non-target tissue and is thus wasted. This reduces the efficiency of the antenna. Antennas with lower efficiency need a higher power setting and/or a longer energy delivery time setting to achieve the same clinical action. Higher power delivery requires the use of larger diameter transmission lines which in turn increases the invasiveness of the procedure. Prior art antennas have tried to use various additional elements located on the microwave device to cut backward coupling. Examples of such elements are chokes, floating sleeves, triaxial construction and baluns. However, such elements are located on the outer surface of the transmission line and thus increase the size of the transmission line. This in turn increases the invasiveness of the procedure.
Also, it is difficult to use a single existing microwave antenna such as a single monopole antenna to ablate tumors that have a thickness or diameter of a few centimeters in a sufficiently short time. For many cancer-related applications, the targeted tumors have an excessive size (e.g. diameter of several centimeters) and a single monopole antenna is of limited use. One of the solutions proposed to increase the lesion size involves using multiple ablation devices simultaneously. This increases the complexity of the ablation system. The overall size and cost of the ablation device is also increased due to more number of elements employed in the system. Also, this increases the invasiveness and complexity of the procedure. Another option to increase the lesion size is to increase the microwave power delivered through the antenna. However, this may increase the temperature of the transmission line of the antenna to unsafe levels thereby increasing the risk of damaging healthy tissue.
Thus there is a need for more efficient microwave antennas that are capable of generating uniquely shaped microwave fields that overcome these problems.